1. Field of the Invention
The present invention relates to an actuator which acts as a light deflector, and includes a plate member, such as a mirror, operating on a substrate, and provides predetermined functions with the plate member in operation, a method of producing the actuator, an optical system using the actuator, and an image forming device.
2. Description of the Related Art
Various kinds of light deflection methods, light deflection devices, and methods of fabricating the light deflection devices have been proposed. For example, Japanese Laid-Open Patent Application No. 2004-78136 (hereinafter referred to as “reference 1”) discloses a light deflection device in which a plate member (such as a mirror) without a fixed end is confined in a space, and is inclined with a supporting member at a center by an electrostatic attractive force to deflect incident light in one axial direction or two axial directions.
In other light deflection devices disclosed in reference 1, a contacting voltage is applied to the plate member (for example, the mirror), or, the plate member is electrically floating (held up by electrostatic force). Reference 1 also discloses light deflection methods of the light deflection devices, respectively, namely, methods for driving the respective light deflection devices.
A typical structure of the light deflection device and a typical driving method thereof disclosed in reference 1 are briefly described below.
FIG. 16A through FIG. 16D are a plan view and cross-sectional views illustrating a structure of a light deflection device disclosed in reference 1.
Specifically, FIG. 16A is a plan view of the light deflection device, FIG. 16B is a cross-sectional view along an AA′ line in FIG. 16A, FIG. 16C is a cross-sectional view along a BB′ line in FIG. 16A, and FIG. 16D is a cross-sectional view along a CC′ line in FIG. 16A. Note that the light deflection device shown in FIG. 16A through FIG. 16D corresponds to one actuator of a two dimensional light deflecting array including plural actuators.
As shown in FIG. 16A through FIG. 16D, the light deflection device includes a substrate 101, plural regulation members 102, a supporting member 103, a plate member 104, and plural electrodes 105a, 105b, 105c, and 105d. 
The regulation members 102 are arranged at corners of the substrate 101, respectively, and each of the regulation members 102 has a stopper at its top.
The supporting member 103, which is arranged on the surface of the substrate 101, has a top part formed of a conductive material.
The plate member 104 does not have a fixed end, namely, the plate member 104 is not fixed. On the upper portion of the plate member 104 there are a light reflecting area and a conductive layer formed from a member with at least a conductive part. At the bottom of the plate member 104 at least a contacting part, which is in contact with the top of the supporting member 103, is formed of a conductive member. The plate member 104 is movably arranged in the space surrounded by the substrate 101, the supporting member 103, and the stoppers of the regulation members 102. The voltage on the plate member 104 is supplied through contact with the supporting member 103.
The electrodes 105a, 105b, 105c, and 105d are arranged on the substrate 101, and are substantially opposite to the conductive layer of the plate member 104.
In the above light deflection device, because of a combination of voltages applied on the electrodes 105a, 105b, 105c, 105d, and the supporting member 103, as shown in FIG. 16C and FIG. 16D, the plate member 104 is inclined in four directions. The four directions are indicated as “direction 1”, “direction 2”, “direction 3”, and “direction 4” in FIG. 16C and FIG. 16D. Therefore, for example, when a light beam is incident in a direction perpendicular to the surface of the substrate 101, the plate member 104 reflects the incident light beam in the four directions. On the contrary, when four light beams are incident respectively in the four directions, the plate member 104 reflects the incident light beams in the direction perpendicular to the surface of the substrate 101.
FIG. 17 is a table illustrating a relationship between the combination of the voltages applied on the electrodes 105a, 105b, 105c, and 105d and the inclination direction of the plate member 104 in the light deflection device in FIG. 16A through FIG. 16D.
As shown in FIG. 17, two voltages, specifically, a voltage of X volts and a voltage of 0 V, are appropriately applied on the four electrodes 105a, 105b, 105c, and 105d, and the supporting member 103. Due to this combination of the applied voltages, the plate member 104 is able to reflect the incident light beam in the four directions.
The light deflection device disclosed in reference 1 has the following advantages.
Since the inclined angle of the plate member 104 is determined by contact between the substrate 101, the supporting member 103, and the plate member 104 (such as a mirror), it is easy to stably control the deflection angle of the mirror.
Since different voltages are applied on opposite electrodes with the supporting member 103 as a center, the thin plate member 104 can be rotated at high speed so that the response speed of the light deflection device is high. Further, since the plate member 104 does not have a fixed end, twisted deformation or other deformation does not occur in the plate member 104, and there is little long-term degradation of the plate member 104; hence the plate member 104 can be driven at a low voltage.
Since a tiny and lightweight plate member 104 can be fabricated by semiconductor processes, there is little shock caused by impact between the stopper of the regulation members 102 and the plate member 104; thus there is little long-term degradation of the plate member 104.
In addition, by appropriately determining the structure of the regulation members 102, the plate member 104, and the light reflecting area, it is possible to improve the ratio of the reflected light in an ON condition and the reflected light on an OFF condition, that is, an S/N ratio in a still image device, or a contrast ratio in a video device.
Since semiconductor processes and semiconductor process apparatuses can be used to fabricate the light deflection device, it is possible to reduce the size of the device and increase the degree of integration at low cost.
Further, since the electrodes 105a, 105b, 105c, and 105d are arranged with the supporting member 103 as a center, it is possible to perform one-axis two-dimensional light deflection and two-axis three-dimensional light deflection.
As described above, the light deflection device disclosed in reference 1 deflects the incident light by inclining the plate member 104, which does not have a fixed end, has many advantages as described above, and is superior to other optical switches, such as a diffraction grating optical switch.
However, a plate member 104 without a fixed end suffers from the following problems.
FIG. 18A through FIG. 18C are a schematic plan view and schematic cross-sectional views illustrating components relevant to contacting the plate member 104 in the light deflection device disclosed in reference 1.
Specifically, FIG. 18A is a plan view illustrating the supporting member 1031, the plate member 104, and the electrodes 105a, 105b, 105c, and 105d of the light deflection device.
FIG. 18B is a cross-sectional view of a portion F along a GG′ line in FIG. 18A.
FIG. 18C is a cross-sectional view of a portion F along an II′ line in FIG. 18A.
In FIG. 18B and FIG. 18C, there is an insulating film 106 for preventing electrical short circuit between the plate member 104 and the electrodes 105a, 105b, 105c, and 105d. For example, the insulating film 106 is formed of a silicon oxide or a silicon nitride film. It should be noted that the insulating film 106 is omitted in FIG. 16A through FIG. 16D.
As shown in FIG. 18B and FIG. 18C, when the plate member 104 is inclined, the plate member 104 contacts the insulating film 106 at a line (line contact), which corresponds to an edge of the plate member 104. Due to this, a sticking force (indicated by open arrows in FIG. 18C) corresponding to the surface energy of the insulating film 106 in contact with the contacting part of the plate member 104 occurs. This sticking force impedes the inclination of the plate member 104, and when an electrostatic force is imposed additionally to cancel out this sticking force, the driving voltage ends up being increased.
In other words, in the light deflection device disclosed in reference 1, the plate member 104 contacts the substrate 101 (specifically, the insulating film 106 of the substrate 101) at a line (line contact), which corresponds to an edge of the plate member 104, and this causes an increased driving voltage. The sticking force can be ascribed to a water bridge force, a molecular force, or electrification. In addition, in reference 1, the contact line between the plate member 104 and the substrate 101 extends nearly the whole length of the plate member 104, and due to this, as described above, contact between the plate member 104 and the substrate 101 causes a relatively large sticking force, and this results in an increase of the driving voltage.
In order to prevent the increase of the driving voltage, for example, one can attempt to change the shape of the contacting part and reduce the size of the contacting part between the plate member 104 and the substrate 101. For example, the contacting part can be formed to have a projecting shape, and such a projecting shape can be fabricated by common semiconductor fabrication techniques, such as lithography and etching. In this case, a processing limit of a pattern, namely, the limiting dimension of a pattern under semiconductor processing, determines the minimum size of the projecting contacting part, and this processing limit is in turn determined by the lithography technique. The processing limit of the lithography technique depends on resolving power or resolution of a resist serving as a mask, whether unevenness exists in the device, and performance of a lithography apparatus, such as a stepper. Currently, the prevailing semiconductor processing technique is the sub-micron or half micron processing technique, and with this technique the processing limit of the projecting shape is about 0.5 to 1.0 μm in line width. This processing limit of processing the projecting contacting part shape depends on the lithography technique; hence, with existing techniques, the reduction of the size of the projecting contacting part is limited.
When using the light deflection device disclosed in reference 1 as an actuator, it is expected that the plate member, which is an operating part of the device, will be inclined while not being fixed. Here, a crucial technical problem is how to reduce the size of the contacting part to a level below the processing limit of the semiconductor processing technique.
Specifically, usually, a light deflection device is applied to an image projecting display device. In recent years, the image projecting display device has been widely applied to rear projection television sets or digital theaters, and it is required to further improve the contrast ratio of the image projecting display device. In this case, it is required to improve the ratio of reflected light in an ON condition and reflected light on an OFF condition. For example, in a DMD produced by Texas Instrument Co. or a mirror type optical switch like the light deflection device disclosed in reference 1, it is necessary to increase the light deflection angle. In other words, it is necessary to increase the inclined angle of the plate member (such as a mirror). Due to this, it is necessary to increase the distance between the plate member and the electrodes facing the plate member. Because the electrostatic attracting force is inversely proportional to the square of the distance between the electrodes, where the electrostatic attracting force occurs, it means the electrostatic attracting force acting on the plate member is reduced in order to improve the contrast ratio. When the sticking force induced in the contacting area of the projecting contacting part, which is fabricated at the processing limit, is predominant over the reduced electrostatic attracting force, the plate member is fixed but cannot be inclined. Thus, the driving voltage has to be increased to solve this problem.